Field
Embodiments described herein relate to determining a position of point and lines sources in a Positron Emission Tomography (PET) apparatus.
Background
The use of PET is growing in the field of medical imaging. In PET imaging, a radiopharmaceutical agent is introduced into an object to be imaged 15, shown in FIG. 1, via injection, inhalation, or ingestion. After administration of the radiopharmaceutical, the physical and bio-molecular properties of the agent will cause the agent to concentrate at specific locations in the human body (i.e., object 15). The actual spatial distribution of the agent, the intensity of the region of accumulation of the agent, and the kinetics of the process from administration to eventually elimination are all factors that may have clinical significance. During this process, a positron emitter attached to the radiopharmaceutical agent will emit positrons according to the physical properties of the isotope, such as half-life, branching ratio, etc.
The radionuclide emits positrons, and when an emitted positron collides with an electron, an annihilation event occurs, wherein the positron and electron are destroyed. Most of the time, an annihilation event produces two gamma photons at 511 keV traveling at substantially 180 degrees apart which are detected by a pair of crystals. By drawing a line between centers of a pair of crystals 10, i.e., the line-of-response (LOR), or drawing a polyhedron formed by connecting corresponding corners of a pair of crystals 10, i.e., tube-of-response (TOR), one can retrieve the likely location of the original disintegration. While this process will only identify a line (or tube) of possible interaction, by accumulating a large number of those lines (or tubes), and through a tomographic reconstruction process, the original distribution can be estimated. In addition to the location of the two scintillation events, if accurate timing (within few hundred picoseconds) is available, a time-of-flight (TOF) calculation can add more information regarding the likely position of the event along the line (or tube).
The above-described detection process must be repeated for a large number of annihilation events. While each imaging case must be analyzed to determine how many counts (i.e., paired events) are required to support the imaging task, current practice dictates that a typical 100-cm long, FDG (fluoro-deoxyglucose) study will need to accumulate several hundred million counts. The time required to accumulate this number of counts is determined by the injected dose of the agent and the sensitivity and counting capacity of the scanner.
Briefly, the PET reconstruction process finds the amount and the location of isotopes (unknown) in the patient from the data recorded in the PET system (known). One of the basic questions in the PET reconstruction process is to find detection probability, which represents the probability of a photon emitted from a voxel that can be detected by a given pair of crystals 10.
For many calibration and analysis modes, a point source located at the iso-center of a PET scanner is required or a line source aligned to the scanner's cylindrical axis and centered radially is required. To determine the location of the source, conventional methods acquire data for a long period of time (ranging from tens of seconds to minutes) and perform image reconstruction. Thus, conventionally it takes a great deal of time to acquire and calculate the location of the point source or the line source.